High strength axle for change up flywheel?

Do you guys think its necessary to have a high strength axle on the flywheel for this years game?



Not even close. I don’t think you should ever need a high strength axle for a flywheel. Unless you have it under an immense amount of pressure which I don’t think that would happen.


Yeah I don’t think will have our flywheel run at a high RPM, which won’t be under too much pressure. The only time it might be, is when you do a full court shot and we might be able to do.

No you will be fine the whole time. Just make sure to space out every thing correctly and you should be fine.

Yea it should be fine with only a normal shaft.

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like Skynet said you should be fine with just a normal shaft

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No it’s not necessary. As long as you build it secure (like anything else) it will be really good.

Depends on two things: the length the axle will span and the load on the flywheel. If the axle spans length is more the full length of the ball (6"+), then use a high strength axle. If your flywheel requires a high amount of compression to transfer energy, use a high strength axle.

I am confused why everyone was so quick to dismiss the idea. For reference, most (if not all) of the 169 teams in TP used HS axles on their world robots. Any minute improvement to the robot is worth it, and a HS axle is simply more durable than a LS one.

So (imo), take the extra effort to get a HS axle to work. In the long run, it will pay off. The last thing you want is right before finals, your flywheel axle bends. The probability might be low, but anything is possible (as history has shown time and time again). Take into account every possibility and work to mitigate it.


Uh… video or it didnt happen

You’d be surprised how easily an axle can bend. Pressure is defined as Force/Area . The axle only experiences one contact point with the ball (because of the spherical nature of the ball) meaning the area will be incredibly small.

This means the axle will experience high amounts of pressure (repeatedly over time) and the axle also needs to be long (depending on the flywheel set up). The current meta that has been shown on the forums has multiple sprockets with tank-tread meaning the axle will have to span a rather large distance.

The last thing you want on a flywheel is a bent axle, and if there’s one thing I’ve taken away from my time in vex (in terms of build quality), axles are very bad . I can bend a low strength axle with my bare hands and with minimal force. That’s a general rule of thumb that shows that that part should be reinforced. The problem with reinforcing an axle is it will cause immense amounts of friction which is the last thing you want on a flywheel.

Additionally, what will it realistically cost you to use a HS axle? A couple of extra minutes drilling out a hole? What will it cost you to use a LS axle? There are many many downsides that I can list. Did I mention LS axles have a tendency to TWIST under high loads. Flywheels have a lot of momentum.


HS shafts tend to have lots of friction due to the fact that there is so much extra contact area in the bearings, and possibly the metal if not drilled right. They are also rather heavy, and will take longer to speed up, combined with the increased friction. Overall, I would go LS axles, and if something bends, just swap it out, it shouldnt be too hard

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As everyone has said they’re heavy and add extra friction. As long as you watch your build quality you shouldn’t need the extra strength on a flywheel.

This makes intuitive sense, and would seem to explain why low strength axles tend to bend when used over very long distances, but thinking about it in terms of pressure is not exactly the best way to go about explaining it, because the pressure on the ball need not be the same as the pressure on the axle. Furthermore, pressure is not the defining characteristic that an intake with good “compression” would need. It’s actually simply just the force on the ball that matters for that, as the normal force the intake puts on the ball is directly proportional to the frictional force that moves the ball. Spreading the force out over a larger area can help stabilize the ball by increasing the necessary torque to turn the ball; however, this is typically a quality of the intake system itself rather than the relative axle strength.

The reason low strength axles tend to bend so much is related to what you said about the distance between the supports of the axle. If you analyze the torque generated about one of the supports (which essentially will act as a pivot) you will see that a very low force can create quite a high torque on the axle because of how far away the supports are from where the force is being applied. Even then, I see a lot of people saying that low strength axles are more than strong enough for the task. In my opinion, this is not the case at all. The way the axle is going to resist bending is by creating a counter-torque to act against the one created by the force on the intake. However, the axle cannot simply generate this torque. This torque is created by the tensile strength of the steel axle. We know that steel has a very high tensile strength, but the amount of this that is turned into a counter torque is related to the sin of the angle that the axle has bent already. This means two things:

  1. The counter torque generated is very small. For small angles, we can approximate sin(theta) as theta, and this shows real quick that the amount of the tensile strength that is turned into a counter torque is going to be an incredibly small fraction of the tensile stress the axle is under.
  2. The second thing we can observe from the fact that the torque is proportional to the sin of the angle created is that when there is no bend in the axle (theta=0), there is quite literally no restoring torque created. This means that the axle must bend a little to balance the torque on it.

The physics of having widely spaced supports on a low strength axle is not at all in favor of low strength axles being strong enough to resist bending noticeably. You’re much better off using high strength axles that can create a high restoring torque even with low angle displacement.


Thank you that is what I was trying to get to

just make sure you keep the shaft as short as you can, not 6" across and it should be fine.

Not necessarily true. It’s a square spinning in a circular hole. The same amount of contact area as a low strength axle but arguably less friction force because the force from the axle is multiplied radially (think of what torque is). So if anything, a HS axle will have an easier time overcoming the force of friction from the bearing than a LS axle. Whether or not you can properly mount a HS bearing is another question (they are very tricky to align properly because of the lack of divots).

They don’t necessarily add extra friction, there are only four contact points with the same surface area as a LS axle (just expanded out radially). They do however weigh significantly more which may not be a bad thing for a flywheel (depending on whether or not you want it to spin the whole match).

There comes a point where build quality can’t outweigh physics (due to the nature of vex parts).

Newton’s third law, the force the ball imposes on the axle/wheel, translates down to the load the axle bears. I agree with you that pressure isn’t the best way to describe the force on the axle as it is typically used to describe the way a gas acts on a surface or larger (more measurable areas). But to be fair, the people above decided to talk about it in terms of pressure.

In TP and NBN, LS axles weren’t an issue because the balls were significantly smaller and “squishy” (for NBN). However, the balls in this game are huge, and they don’t compress and neither should the hood of the flywheel (to an extent) which means two things: a) the axle will bend or b) the ball won’t move.


I don’t see what’s wrong with the hood compressing, I feel like that might be the meta. Personally I don’t think a HS shaft is necessary for a flywheel if you build it well and it isn’t doing crazy shots (just tossing) then a normal shaft should be fine. But if you have doubts, a HS shaft isn’t going to hurt.


In this case Newton’s Third Law only tells us that the force on the axle due to the ball and the force on the ball due to the axle are the same. The linking piece of tread is the missing element that makes the pressure on the ball less because there is more contact area with the ball and the tread than there is between the tread and axle.


Ahh I see what you’re saying. So yes, the ball does put on a significant pressure on the tank tread. But it gets dispersed by the sprocket on the axle. That actually makes a lot of sense.

As for dispersing the pressure of the ball on the tank tread, you could wrap the flywheel in foam, and then wrap that in rubber bands. This would ensure the foam cannot come off, and the foam would disperse the pressure from the ball.

In conclusion, whether or not you want a HS axle totally depends on the width of the flywheel. If you want to be safe (logical /s), use one regardless.

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